A current world-wide obesity epidemic also carries a high morbidity due to associations with heart disease, hypertension, diabetes and stroke. The central tenant of this proposal is that a genetically-determined reduction in the leptin sensitivity of specialized brain metabolic sensing neurons causes genetically predisposed individuals to develop diet-induced obesity (DIO) when supplied with sufficient dietary calories and fat. Using rats selectively bred in our lab to express the DIO vs. diet-resistance (DR) traits on a 31% high energy (HE) diet, we will test this hypothesis using an integrated behavioral, physiological, molecular and neuroanatomical approach at the in vivo and in vitro levels. Hypothesis I proposes that reduced leptin sensitivity is reflected in functional imbalance between two of its important downstream neuropeptide effector systems, the anabolic neuropeptide Y (NPY) and the catabolic melanocortin, alpha-melanocyte simulating hormone (alpha-MSH). This imbalance leads to a net anabolic response to HE diet in metabolic, behavioral and physiologic systems controlled by them. We will characterize the integrated responses of DIO vs. DR rats to HE diet exposure and to IIIrd ventricular (3v) infusions of leptin, NPY and melanocortin agonists and antagonists using molecular (real-time PCR), neuroanatomical (immunocytochemistry, in situ hybridization, tract tracing), biochemical (plasma glucose, insulin, leptin, glycerol), physiological (brown adipose temperature, sympathetic activity), cellular (neuronal calcium imaging, single cell PCR) and behavioral (food intake, motor activity) methods. Further, we will use drugs which alter leptin receptor function to assess the effect on the physiological responsiveness of DIO and DR rats at the whole animal and cellular level. Hypothesis II proposes that maternal obesity in DIO dams increases offspring obesity by further reducing leptin sensitivity, while voluntary wheel running exercise will prevent the onset of obesity by selectively reducing adipose stores in young DIO rats by increasing their leptin sensitivity and the function of its downstream neuropeptide effector systems. This hypothesis will be tested using similar methodologies to those used in Hypothesis I in offspring of obese and lean DIO and DR dams and in exercising juvenile DIO and DR rats. The overall goal of this proposal is to identify sites within the leptin signaling pathway at which pharmacological or other interventions might be developed to prevent or treat obesity in humans.